The present invention relates to an improved process and apparatus for electrolytically producing alkaline chlorate wherein effective auto-oxidation of hypochlorous acid is attained thereby causing the current efficiency of the electrolysis to be improved.
The conventional process of electrolytically producing alkaline chlorates from an aqueous solution of alkaline chloride includes two main steps, that is, an electrolysis step and an auto-oxidation step. In the electrolysis step, chlorine ions in the alkaline chloride solution discharge electrons at an anode, whereby hypochlorous acid ions are produced, while hydrogen ions are reduced at a cathode to form hydrogen gases. In the auto-oxidation step, the produced hypochlorous acid is auto-oxidized to form chlorates.
However, the above process is accompanied by sub-reactions which lead to a decrease in the current efficiency. In the sub-reactions, some of the produced hypochlorous acid is oxidized by the discharge of electrons at the anode to form chlorates with oxygen being generated, while the remaining hypochlorous acid is reduced back to chlorine ions when the oxygen is removed at the cathode.
In order to prevent such disadvantageous sub-reactions from occurring, adjustment of the pH value of the solution, utilization of improved anode materials, addition of chromates and so on have been attempted in conventional electrolysis processes. However, using such methods, a current loss (decrease in the current efficiency) of about 3.about.8% occurs at the anode and a current loss of 2.about.4% occurs at the cathode.
In general, apparatuses for electrolytically producing alkaline chlorate have been used, wherein the electrolysis, auto-oxidizing reaction, gas-separation and cooling are effected concurrently within a single unit. Such a single unit is referred to as a "box type" electrolysis vessel. Such box type apparatuses are relatively flat with a large bottom surface area, and because of this a back mix flow reaction is likely to occur causing a low efficiency of the electrolytical production. Further, it is necessary that the electrolysis operation be carried out at a reduced or low temperature in order to protect the apparatus against corrosion. This results in a great amount of electric power being consumed.
There have been attempts or suggestions to improve the process and apparatus for the electrolytical production of alkaline chlorate. However, insofar as is known, there is no process or apparatus which is satisfactory on an industrial scale. For example, referring to U.S. Pat. No. 3,539,486 corresponding to Japanese patent application (Publication No. 51-30035), an apparatus which may be referred to as "a pipe line type apparatus", is disclosed comprising an electrolysis vessel and a reaction vessel connected to each other by conduit pipes, wherein a solution is forced to circulate by the lifting force of the hydrogen bubbles released during the electrolysis. The disclosed apparatus has many disadvantages. Since there are many pipe joints, there is a problem of leakage of the solution from the pipe arrangement. The electrolysis vessel and the reaction vessel are arranged spaced apart from each other, which leads to an increased area of floor space being occupied by the apparatus. With such a pipe arrangement, corrosion is likely to occur and, thus, it is necessary that the apparatus be made of expensive anti-corrosive materials, such as titanum, teflon and the like.
Another published Japanese patent application (Publication No. 52-30960) discloses an apparatus comprising a box type reaction vessel in which an electrode cell is mounted onto an inside wall surface of the vessel and a vertical partition, surrounding the cell on the wall is also mounted onto the inside wall surface of the vessel to separate the vessel space into two sections. A solution in the vessel is forced to circulate between the two space sections separated by the partition. Such an arrangement does not allow the vessel to contain a great amount of the solution therein, and leads to a non-uniform circulation of the solution. This results in the reaction space defined by the vessel and the partition, not being fully utilized to effect the necessary reaction. Further, the apparatus has disadvantages in that the base of the apparatus is relatively large, since it is of a box type of a relatively flat form. Still further, the electrolysis operation cannot be carried out at an increased temperature, since the apparatus is constructed of a hard polyvinyl chloride or the like.
In the above mentioned two kinds of conventional apparatuses, that is, a pipe line type and a box type, the electrolysis temperature is still low, that is, 60.degree. to 80.degree. C., and, thus, the advantages of a high temperature operation cannot be enjoyed. Also, the apparatuses still have to be equipped with a cooling system, which causes the apparatuses to have a complicated structure.